Here is a simple over voltage protector circuit that breaks the power to the load if the voltage increases above the preset level. The power will be restored only if the voltage drops to normal level. This kind of circuit is used in voltage stabilizers as over load protection.
The circuit uses a Zener controlled relay driver to cut off the power in the over voltage condition. A 0-9 volt transformer, diode D1 and the smoothing capacitor provides 9 volts DC for the circuit. Any voltage increase in the Primary of the transformer (as the mains voltage increases) will reflect as a corresponding voltage increase in its secondary also. This principle is used in the circuit to trigger the relay. When the input voltage to the primary of the transformer (around 230 volts), Zener will be out of conduction (as set by VR1) and the relay will be in the de- energized condition. The Load will get power through the common and the NC contacts of the Relay. In this state, LED will be off.
When the voltage increases, Zener diode conducts and the relay will be activated. This breaks the power supply to the load. LED shows the activation status of the relay. Capacitor C1 acts as a buffer at the base of T1 for the smooth working of T1 to prevent relay clicking during its activation/deactivation.
Load is connected through the Common and the NC (Normally Connected) contacts of the relay as shown in the diagram. The phase line is connected through the relay contacts. Neutral should go directly to the load.
Before connecting the load, slowly adjust VR1 till LED just turns off assuming that the lines voltage is between 220-230 volts. If necessary, check the line voltage using an AC volt meter. The circuit is ready for use. Now connect the load. When the voltage increases, Zener will conduct and actuates the relay. When the lines voltage returns to normal, again the load will get power
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